Increasing concern over the reliability and availability of nuclear reactor residual heat removal systems has been voiced by government regulatory agencies and industry standards committees. Although nuclear reactors can be shut down in the event of an abnormal incident with a high degree of reliability, the fuel continues to produce residual heat that can cause fuel meltdown if adequate cooling is not available. In current pressurized water reactor (PWR) designs, residual heat removal is accomplished by dumping steam from the steam generators and by supplying makeup feedwater to the steam generators through auxiliary feedwater systems. These auxiliary systems are complex, requiring the proper functioning of such active components as pumps, valves, instrumentation and controls, diesel generators, and switch gear. Another reason for the complexity of current residual heat removal systems is that they have commercial functions in addition to their safety-related functions. For example, such systems have complex control systems to accommodate plant depressurization and cooldown such as occurs prior to refueling. The equipment associated with current residual heat removal is located in diverse areas of the power plant, and the makeup water is stored in tanks outside the reactor containment building. The complexity and accessibility of major parts in current systems raises concerns over reliability and vulnerability to sabotage.